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Excessive load to bearings might cause cracks in the races or the rolling elements themselves. A crack in a bearing race will generate a sharp pulse whenever a roller passes over it. A roller crack will generate a pulse whenever the roller interfaces with the inner or outer race.
Bearing Corrosion
Corrosion is a problem for external bearing, such as those on the tail drive shaft. Corroded bearing races typically produce wide band noise, creating an increase in the total vibration energy.
2.2.3 Rotors
The rotors are the non-redundant lifting and anti-torque devices of a heli-copter. Control and propulsion is also managed by the rotors, making them the most critical part of any helicopter. Serious rotor damage is usually catastrophic.
Unbalance
A rotor must have its Center of Gravity (CG) at the center of the mast to avoid vibration at the frequency corresponding to the mast rotating speed. The amplitude of an unbalance is identi.ed by measuring vibration in the rotor plane. Unbalance direction (which blades are too light or too heavy) can be measured using a blade indexer. Correction is done by simply adding or removing weights from the blades.
Blade Track Split
All the blades of a rotor should ideally follow the same track. Worn blades do however tend to change track slightly, causing increased vibration levels. Blade track can be estimated either by measuring vertical acceleration, or by using a camera measuring the distance between each blade and the airframe. Correction is done by altering length of the blade pitch links, or by using bendable .aps on the blades.
Bearing Wear
Some rotorcraft use fully articulate rotors, meaning that each blade can be rotated along three axis. This is achieved by .xing the blade sleeve to the hub using an assembly of three traditional bearings, or a single spheric elastomer bearing. The former solution is prone to traditional bearing problems, while the latter might su.er from damaged elastomer. Such problems are identi-.able by an increase in vibration energy at the frequency corresponding to the mast rotating speed or blade pass speed.
Damper Wear
Fully articulate rotors use dampers on the main rotor to damp blade move-ment in the lead / lag (horizontal) plane. Worn dampers are identi.able by an increase in vibration energy at the frequency corresponding to the mast rotating speed or blade pass speed.
Swashplate Eccentricity
The swashplate is basically a gigantic bearing which encircles the main rotor mast. One part of the swashplate is .xed to a set of actuators mounted on the top of the main gearbox. The other part is connected to the blade pitch links. The swashplate is the medium allowing the stationary actuators to control the angle of the rotating blades.
Like all bearings, the swashplate is prone to generate some eccentricity after excessive use. This is usually detectable as an increase in vibration energy at the frequency corresponding to the mast rotating speed or blade pass speed.
　
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